This is one of the biggest transfers in Indian football.

In a shocking development in Indian football's scheme of things, former Mohun Bagan midfielder Katsumi Yusa has joined the Mariners' arch-rivals East Bengal earlier today. The Japanese, who has been playing in India since 2011, will join the Red and Golds before the start of the 2017-18 I-League season.

Katsumi's move from Bagan to East Bengal has already sparked mixed reactions across Kolkata; as one half of the Kolkata derby divide celebrates, the other half is in a funereal mood.

Katsumi played 78 times for Mohun Bagan in his four years at the club and even led the club to the I-League title in 2014-15 and the 2016 Federation Cup. He has joined Khalid Jamil's retooled East Bengal on a one-year deal and is expected to be in Kolkata in time for the start of the 2017-18 I-League campaign.

Alongside former Syria international Mahmoud Al Amna, Yusa will be the second Asian player in East Bengal's ranks for the forthcoming I-League campaign, although he will not be available for the ongoing Calcutta Football League season, unlike Al Amna.

Bagan, who released Katsumi after the club's 2016-17 season came to a disappointing end with a loss to Bengaluru FC in the Federation Cup final, have already roped in former Shillong Lajong man and Katsumi's Japanese compatriot Yuta Kinowaki to replace the new East Bengal man.

It isn't common to see players who have played for East Bengal in the past switch Kolkata derby allegiances to join Mohun Bagan and vice versa. Earlier this summer, the Mariners signed former East Bengal defender Abhishek Das after the player was overlooked in the ISL draft.

Katsumi's arrival at East Bengal will significantly improve their chances of landing a first I-League title. The Red and Golds appear to leave no stone unturned in their pursuit of the elusive national title and the arrivals of title winners like Jamil, Al Amna and Katsumi suggest the club are aiming higher than ever before.

East Bengal have made a fine start to their CFL 2017 campaign, winning all three of their opening week fixtures. Jamil and his charges will look to seal the club's eighth straight CFL title before turning their attention to the bigger matter of the I-League, and they seem to have earned an early advantage by signing Katsumi.